STACKING FOR GAIN
The following information is supplied courtesy of Matchmaster
STACKING FOR GAIN
One method of increasing the gain of an antenna system is to stack
more than one of the same type of antenna.
In marginal areas this strategy can be the only way to raise the
signal to noise ratio to a level where effective amplification is
possible.
Antennas can be stacked in either plane, vertical fig.1 and
horizontal fig.2.
|
Figure 1. |
Figure 2. |
 |
 |
| A stacking distance of 0.5 times the wavelength will give a nominal gain increase of 2.5dB. If the boom length of the antenna is greater than 0.5 of the wavelength then a stacking distance equal to boom length will give a gain of approx. 3dB. Reducing the stacking distance to 0.75 times the boom length usually results in a corresponding drop in gain to approx. 2.75dB. |
 |
Stacking for
Interference
Horizontal stacking can be an effective means of reducing adjacent
channel interference. For instance, some Sydney viewers have
difficulty receiving Wollongong channels 53, 56 and 59 in the face
of strong adjacent channel interference from Kings Cross channels
52, 55 and 58.
It can be shown that for a given angle between the desired and
undesired transmitter site, see fig.3, two identical antennas,
horizontally stacked at distance "d", fig. 4, will have equal
signals of opposite phase across their respective terminals. In
theory the resultant signal strength should be zero, however in
practice you may achieve 20dB or, with careful adjustment, 35-40dB.
| Figure 3. | Figure 4. |
 |
 |
| The formula for calculating the stacking distance is: d = | ___l__ |
| 2 Sin q |
| Where | d = stacking distance (m) l = wavelength of interference signal (m) q = angle (°) between wanted and unwanted signals. |
Assuming our original problem, let's suppose we wish to receive ch.
59 Wollongong with adjacent channel interference from ch. 58 at
Kings Cross and, we calculate the angle between the two sites to be
110 degrees.
From the above table we find the wavelength of ch. 58 to be 0.407m.
| Thus d = | 0.407 | = 0.217m or 217mm |
| 1.879 |
The connecting co-axial cables should be of equal length and coupled together using a HL15 high isolation outdoor coupler.
The stacking distance becomes impractical at angles smaller than 20 and greater than 160 degrees. However, as the angle approaches 180 degrees the method outlined in the next article is more appropriate.
STAGGER STACKING
Adjacent channel interference caused by signals from the rear can be
minimised by "stagger stacking", that is, by stacking two identical
antennas horizontally, with one antenna positioned 1/4 wavelength
behind the other. Refer fig. 5.
It can readily be seen that the unwanted signal will appear across the terminals of both antennas except that the waveform at A, the leading antenna, will be lagging by 90° or, 1/4 wavelength behind antenna B. To complete the excercise the two connecting cables are identical also, except that cable "L" from antenna A is 1/4 wavelength longer than cable "l" from antenna B. This causes a further delay of 90° making the total phase difference at the junction of the two cables 180° which, in an ideal world, would mean that the two signals would cancel each other out. The reality is that even with the most careful adjustment of the relative positions of the two antennas and, proper calculation of the relative lengths of the two cables the best attenuation possible would be a not inconsiderable 35-40dB.
| Stagger distance x = | _l_ | Cable differential L-l = | lx Vf | |
| 4 | 4 |
| Figure. 5. |
 |
Where | x = stagger distance l = wavelength of interfering signal L = longer length of cable (antenna A) l = shorter length of cable (antenna B) Vf = velocity factor of the co-axial connecting cable(RG6=82%) |
For the offending channel 58 with a wavelength of 0.407m, the
stagger distance is 102mm and the
L-l differential is 82mm. Therefore if "L" was chosen to be 500mm
then "l" would be 82mm shorter or,
412mm.
Lagging Ghosts - Ghosts on the right hand side of the picture
The majority of ghosting falls into this category and
is caused by transmissions which are reflected off
large objects such as tall buildings, bridges or hills, and thus
travel further and, arrive later. The time
interval between the primary signal and the ghost(s) determines how
far the secondary pictures are
displaced to the right.
If the speed of light is 300Mm per second then it stands to reason
that if a secondary signal arrives at
the antenna lms later than the original transmission then it must
have travelled an extra 300m. If we relate
this to a 48cm receiver which has a physical width of 420mm and an
electrical width of 57.5ms, (64ms
line length less 10% flyback time), then our 1ms delay corresponds
to a ghost image spacing of 7.3mm.
Mismatch between antenna and feeder, or feeder and receiver, may
also be responsible for lagging
ghosts.
Leading Ghosts - Ghosts on the left hand side of the
picture
Usually experienced in areas close to the transmitter.
Can be due to pick up by the receiver chassis, 240V
mains cable, 300 ohm ribbon (on older installations), or even by
poorly shielded co-axial cable.
Prevention of Ghosts
For lagging ghosts the problem can be minimised by
selection of an antenna with higher gain coupled
with a narrower frontal lobe and, perhaps a better front to back
ratio.